Orthopaedic components, in particular femoral hips and knees, are commonly manufactured from alloys of cobalt plus chromium, titanium, or stainless steel. Additionally, the ceramic materials aluminum oxide (alumina) and zirconium oxide (zirconia) are also used for femoral hips. The use of bulk ceramic materials is avoided in applications such as femoral knees, where the prosthesis is subject to tensile forces, since these materials are known to be weak under such conditions. There are numerous studies see R. M. Streicher, "Ceramic Surfaces as Wear Partners for Polyethylene", in Bioceramics, London, ed. by W. Bonfield, G. W. Hastings, and K. E. Tanner, Butterworth-Heinemann, 1991, and E. Doerre, "Retrieval and Analysis of Ceramic Hip Joint Components", Trans. Soc. Biomat. 66, 1988! indicating that an alumina surface causes decreased wear on the companion ultra high molecular weight polyethylene (UHMWPE) articulating surface compared to a metal alloy femoral component. See also U.S. Pat. No. 5,037,438, J. Davidson and "Low Wear Rate of UHMWPE Against Zirconia Ceramic (Y-PSZ) in Comparison to Alumina Ceramic and SS316L Alloy", J. of Biomed. Mat. Res. 25, p. 813 (1991).
There exist numerous well-known methods for depositing a coating of alumina on an arbitrary surface, such as evaporation, sputtering, arc deposition, laser deposition, and chemical vapor deposition. Such techniques are characterized by an abrupt discontinuity in composition and properties across the interface between substrate and coating. Such discontinuity often causes poor adhesion or interfacial stress leading to debonding of the coating. The articulating surfaces of orthopaedic appliances, which are required to function without repair for periods in excess of ten years, rarely are provided with such coatings, mainly due to the risk of debonding or flaking. The flakes or particles are often trapped in the UHMWPE, resulting in a more rapid 3-body wear which further produces even more particles.
The use of high energy ion implantation of various elements is well known for improving the surface properties of metals, particularly decreasing wear and friction. See "Surface Modification of Metals by Ion Beams", Elsevir Sequoia (1984). For alloys containing primarily the elements cobalt and chromium, ion implanted titanium and nitrogen have been successfully employed. See "Friction and Wear Behavior of Cobalt-Based Alloy Implanted with Ti or N", Mat. Res. Soc. Symp. Proc. 27, p. 637 (1984). Similarly for orthopaedic components, see "Medical Applications of Ion Beam Processes", Nuc. Inst. and Meth. in Physics Res. B19/20, p. 204-208 (1987) and U.S. Pat. No. 5,123,924, Sioshansi et al.
Ion implantation is useful as a treatment for femoral orthopaedic components because the ions are imbedded throughout a zone below the surface of the component rather than being deposited onto the surface, thus avoiding the adhesion problem of a discontinuity at an interface. However, ion implantation is normally unable to create a surface layer of useful thickness or in arbitrary molecular form. The lack of surface layers is primarily caused by sputtering, which erodes the treated surface. After a sufficient dose is implanted, the rate of sputter erosion of substrate plus previously implanted atoms becomes equal to the rate of addition of atoms, thus determining a maximum possible concentration of implanted atoms. Even if the rate of sputtering happens to be sufficiently small, thus enabling a sufficient quantity of atoms to be retained, the atoms are unlikely to be in a desired molecular form such as a ceramic, but rather as individual atoms in a mixture or alloy. A method for avoiding these problems for the specific case of zirconia has been described. See U.S. Pat. No. 5,383,934, Armini et al. An alternate method for improving the adhesion of alumina coatings on substrates has also been given. See U.S. Pat. No. 5,045,345, Singer.